Defibrillators have been widely used to administer a high-voltage, direct-current shock pulse to a patient undergoing cardiac arrest occurring because of asynchronous depolarization, i.e., fibrillation, of cardiac cells. When sufficient electrical energy is delivered to the heart from an external defibrillator through two or more electrodes positioned to engage the patient, fibrillation of the cardiac cells may be arrested. Thereafter, synchronous or normal depolarization of the cardiac cells will often resume.
Defibrillators are typically calibrated in terms of the energy level of the defibrillation pulse they generate, i.e., defibrillators are calibrated in terms of joules or watt seconds. Evidence also has been developed which suggests that it may be desirable to calibrate the defibrillation pulse in terms of the peak current delivered by the pulse. As a consequence, defibrillators which calibrate the defibrillation pulse they generate in this manner may become commercially available.
"Energy dose" defibrillators which display the peak current for each defibrillation pulse they provide, after the pulse is delivered, are known. For instance, in U.S. Pat. No. 4,328,808, Charbonnier et al. disclose an "energy dose" defibrillator which displays peak current, transthoracic impedance, and delivered energy for each defibrillation pulse provided by the defibrillator. Additionally, in U.S. Pat. No. 4,506,677, Lambert discloses an "energy dose" defibrillator which includes a circuit for measuring the time-dependent variation of the delivered current of a defibrillation pulse. The Lambert defibrillator inserts a representation of that variation into a recording of the patient's ECG signal after delivery of the pulse.
The Charbonnier et al. and Lambert defibrillators are not designed to display the current which is anticipated to be carried to the patient by the defibrillation pulse. Although it may be interesting to know the magnitude of the peak current carried by a defibrillation pulse after the pulse has been delivered, it is far more useful to know the magnitude of the peak current which is expected to be carried by a defibrillation pulse prior to application of the pulse. Such anticipated current, which will vary as a function of the transthoracic impedance of the patient, may be used by the health care professional operating the defibrillator in determining the placement of, and pressure to be applied to, the defibrillator paddles (electrodes) required to ensure the defibrillation pulse delivers the selected amount of energy to the patient. It is believed that no known "energy dose" defibrillators are designed to permit a user to select the current anticipated to be carried by the defibrillation pulse prior to application of the pulse or to display such selected current. "Current dose" defibrillators are disclosed in U.S. Pat. Nos. 4,574,810 and 4,771,781 to Lerman and in U.S. Pat. No. 4,840,177 to Charbonnier et al. These defibrillators automatically determine the transthoracic resistance of the patient, and use this resistance to calculate the threshold level of peak current required to effect defibrillation. The defibrillation pulse is delivered to the patient based on the calculated peak current so as to ensure the pulse does not exceed the predetermined defibrillation pulse threshold. After the defibrillation pulse is delivered, the transthoracic resistance, peak current, and delivered energy of the defibrillation pulse are displayed.
It is believed that no known "current dose" defibrillators display the energy which is anticipated to be discharged by the defibrillation pulse prior to application of the pulse. Information regarding the anticipated energy to be provided by the defibrillation pulse may be used by the health care professionals administering the pulse, particularly those professionals who tend to think of defibrillation pulses in terms of the energy delivered by the pulse, in optimizing the placement and pressure of the defibrillation paddles.